When small particles are hit by light, refraction patterns of light result, depending on the size of the particles. This phenomena can be used in determining the size of the particles. In practice, monochromatic light is used in the form of a parallel beam which is allowed to strike a specimen cell in which the specimen or sample is located, and throws the diffraction patterns of the specimen onto a screen comprising a light sensitive detector means.
This has a number of annular light receiving elements, the radii of which are chosen according to the diffraction pattern expected so that the energy distribution of the diffraction pattern can be detected and delivered to an electronic analysing device which comprises a computer and calculates the grain size distribution of the respective specimen from the energy distribution detected.
Known apparatus found on the market for determining grain size have the disadvantage that on altering the measuring range, the image lens must be changed. However changing the image lens results in the whole system having to be adjusted, representing an extensive operation.
In a known process and apparatus for counting biological particles (Meric U.S. Pat. No. 3,830,569) two samples of blood cells are successively placed into the trajectory of the converging light beam at a distance D and D/1.8 respectively to obtain an analysis of blood particles. However, such system is not adapted to analyse grain size distribution patterns of mineral particles.